This invention relates to a receiver having a synchronizing circuit with a variable bandpass filter controlled by an automatic gain control signal.
When a conventional television receiver receives off-the-air signals, such as produced by a video tape recorder (VTR) or by cable television, a flagging or out-of-sync operation may occur because such off-the-air signals tend to have irregular sync pulses. As a result, the television receiver tends to show trace-shifts during portions of the field scan. This phenomenon is called flagging and is exhibited in the television picture as the bending of vertical lines in random fashion. This condition occurs because the passband of the filter in the horizontal phase control circuit is conventionally selected to be narrow in order to improve noise immunity during weak or normal broadcast signal reception.
To eliminate flagging or out-of-sync operation when a video tape recorder is to be utilized, it has been known to provide an external switch which converts the anti-hunt circuit in the horizontal oscillator from a narrow passband to a wide passband, such as shown in Willis U.S. Pat. No. 3,795,762. It would be desirable to provide automatic rather than manual modification of the deflagging circuit so as to eliminate any out-of-sync operation, whether caused by use of a VTR or by cable TV. It also would be desirable to not have to program the receiver in order to detect the particular channels which might receive cable TV signals and VTR signals.
Prior television receivers have automatically controlled the bandpass filter in the automatic phase control circuit in order to switch between narrow and wide passbands. For example, Richman U.S. Pat. No. 2,848,537 and Pollak U.S. Pat. No. 2,962,666 show APC circuits in which the passband is switched from narrow to wide in response to an out-of-sync condition. However, an out-of-sync condition has no correlation with the problem of off-the-air signal reception, and thus is not pertinent to the problem to which the present invention is directed.
It has been known in transistor circuits to provide an automatic gain and bandwidth control in order to minimize frequency shift. For example, Burger U.S. Pat. No. 2,774,866 shows a transistor circuit for a radio receiver in which an AGC signal varies the bandwidth of an amplifier to vary its damping and prevent the Q from rising due to an unloading action. However, this problem is not analagous to the problem of off-the-air signal reception in a receiver.
In a pending application Ser. No. 432,965 of Gregg et al., filed Jan. 14, 1974, entitled "Rate Modifier for a Television Deflection System", and assigned to the same assignee as the present application, automatic switching of the bandpass filter in the horizontal APC circuit from a narrow to a wide bandwidth has been accomplished under control of the vertical sync pulse. In particular, the detected presence of the vertical sync signal enables a switch which disables the noise filtering circuit in the horizontal AFC stage for a predetermined number of horizontal lines. This corrects for irregular sync pulses which tend to be generated by a VTR during the beginning of each vertical scanning period. However, such a system would not correct for irregular sync which occurred later in a field scan.